1. Technical Field
Embodiments of the invention relate generally to power converters. Other embodiments relate to power switch health monitoring.
2. Discussion of Art
Trains typically feature a number of cars that are pushed or pulled by a locomotive. The locomotive has traction wheels engaged with the track. In modern designs, electric wheel motors drive the traction wheels. The electric wheel motors are powered via electrical distribution from one or more engine-driven generators housed within the locomotive. The traction wheels and wheel motors can be reversibly configured, to also act as brakes for slowing the locomotive.
Similarly, in the mining industry, off-highway vehicles (“OHVs”) usually employ electrically motorized wheels for propelling or retarding the vehicle. In particular, OHVs typically include a large horsepower diesel engine in conjunction with an alternator, a main traction inverter, and a pair of wheel drive assemblies housed within the rear tires of the vehicle. The diesel engine is directly associated with the alternator such that the diesel engine drives the alternator. The alternator powers the main traction inverter, in which semiconductor power switches commutate the alternator output current to provide electrical power to electric drive motors of the two wheel drive assemblies.
In both locomotive and OHV applications, solid state power converters are used to provide high voltage current from the generators or alternators to the wheel motors. Such power converters include inductive coils to step down the voltage as well as semiconductor power switches to commutate the current. Although the above-described applications are typical, it will be appreciated that power converters can be used in many other settings.
Generally, operation of a power converter is accomplished by applying alternately two different gate voltage levels to individual semiconductor power switches via corresponding gate drive units. It is a known problem that power semiconductor devices, i.e., “power switches,” are subject to cyclic thermal stresses. While forward biased, each semiconductor conducts significant current at a relatively small voltage drop across the device. Despite the relatively low voltage across the forward-biased semiconductors, resistive heating nonetheless occurs. When forward bias or gate voltage is removed, each semiconductor ceases to conduct. Thus, in the absence of excessive leakage current, ungated power switches should cool toward ambient temperature.
Although durability is a consideration in semiconductor design, electrical design constraints entail that the various layers of the power switches are fabricated from materials having differing thermal properties; in particular, differing coefficients of thermal expansion. Accordingly, over time, thermal stress can cause a mechanical failure such as delamination, debonding of terminals, or fatigue cracking. Thermal stress also can cause an electrochemical failure such as current filamenting and Kirkendall void formation.
Operational stressors such as thermal cycling limit the useful lifetime of a power switch due to degradation of switch health through debonding or delamination. If switch lifetime is not accurately known, it will be replaced either too early, which leads to unnecessary costs and interference with desired operation, or too late, following a failure in operation. The actual remaining lifetime of a switch is hard to predict without measurement information. Typically, measurements used for predicting lifetime are taken only during shutdown or maintenance cycling of the electronic system, e.g., power converter, etc., in which a power switch is installed. As will be appreciated, shutdown or maintenance cycling interferes with desired operation.
Therefore, it is desirable to accurately determine the remaining lifetime of a power switch in operation.